Exercise 6 Open-Loop Speed Control EXERCISE OBJECTIVE To understand what is open-loop speed control; To learn how to sense the speed of the trainer Bidirectional Motor; To control the speed of the trainer Bidirectional Motor in the open-loop mode; To describe the effect of a load variation on the motor speed in the open-loop mode. DISCUSSION Open-Loop Speed Control Systems An open-loop speed control system is a system in which the speed of the actuator is controlled by a setpoint only, and the actual actuator speed is not taken into account. Figure 6-1 shows the block diagram of an open-loop speed control system, using a valve of the flow type: The "setpoint" corresponds to the desired actuator speed; The "valve" provides air flow to the actuator to bring its speed to the setpoint; "Disturbances" are varying conditions that cause the actuator speed to differ from the setpoint; The "actuator speed" is a function of the flow rate allowed by the valve and the disturbance(s). Figure 6-1. Open-loop speed control system. The setpoint is initially set so that the flow rate allowed by the valve maintains a certain actuator speed. This speed will be maintained under constant actuator load 6-1
and air supply. If, however, one of these parameters varies, the actuator speed could change. Because the system is operating in the open-loop condition, it does not receive feedback to correct for changes in actuator speed. The operator may have to readjust the setpoint continuously to maintain the actuator speed at a desired value, which is a time-consuming task and may provide imprecise results. Open-Loop Control of Motor Speed In applications where the air supply and the load on the motor remain fairly constant after the motor has been adjusted to the desired speed, an open-loop speed control system will provide satisfactory results at low cost. If, however, the motor load varies from time to time and good speed regulation is of great importance, most likely the open-loop speed control system will be inappropriate. Sensing the Speed of the Trainer Bidirectional Motor As Figure 6-2 shows, the frequency-to-voltage converter of the Signal Conditioners module can be used together with the trainer Diffuse Reflective Photoelectric Switch to sense the speed of the pneumatic motor. To do so, the photoelectric switch must be positioned perpendicularly to the motor at a distance of 10 cm (4 in). The beam of the photoelectric switch must be pointing in the direction of the white sticker on the motor shaft, and its electrical contact must be connected to the input of the frequency-to-voltage converter. 6-2
Figure 6-2. The trainer Signal Conditioners. As the motor rotates, the electrical contact of the photoelectric switch alternately opens and closes, thereby producing a 0-24 V pulsed signal whose frequency is proportional to the motor speed. This frequency is converted into a proportional DC voltage by the frequency-to-voltage converter. The higher the motor speed, the higher the voltage produced by the frequency-to-voltage converter. The frequency-to-voltage converter has three different outputs, labeled N, 0-5 V, and 0-10 V : The output N has a fixed calibration. It provides a voltage of 1.0 V per 1000 r/min of the motor; The two other outputs, labeled 0-5 V and 0-10 V, can be user-calibrated using the potentiometer S (span). This potentiometer sets the converter input frequency, and therefore the motor speed, for which the voltage will be maximum at the 0-5 V or 0-10 V output of the converter. 6-3
Figure 6-3. Sensing the speed of the trainer Bidirectional Motor. If, for example, the span potentiometer is adjusted to obtain 10.0 V at the 0-10 V output of the converter when the input frequency is 33 Hz (2000 r/min), the output voltage will be 7.5 V if the input frequency is reduced to 24 Hz (1500 r/min). The span setting also determines the required setpoint voltage for a particular point of operation. In this example, the setpoint voltage must be set at 7.5 V to call for a speed of 1500 r/min (24 Hz). 6-4
Controlling the Motor Speed Using a Servo Control Valve of the Pressure Type As seen previously, servo control valves of the pressure type are designed to control the pressure in a pneumatic circuit, not the flow. Unfortunately, this is the flow rate which must be set to control the speed of a pneumatic motor. It is therefore necessary to add a valve, whose orifice varies depending on a pressure level, between the servo control valve of the pressure type and the pneumatic motor. This can be done by means of a proportional control valve as shown in Figure 6-4. Detailed valve operation is as follows: When the pressure at the pilot port (output pressure of the servo control valve) is zero, the proportional control valve is at rest position and no air flows through it (the valve is non-passing); When the pressure at the pilot port (output pressure of the servo control valve) starts to increase, the valve becomes actuated and air starts to flow through its orifice. The opening of the proportional control valve varies in proportion with the pressure level applied to its pilot; The opening of the proportional control valve is maximum when the pressure applied to its pilot is maximum. Figure 6-4. The trainer Proportional Control Valve, Air-Pilot Operated module. Procedure summary In the first part of the exercise, Setting Up the Equipment, you will set up the equipment. In the second part of the exercise, Positioning the Pilot of the Proportional Control Valve, you will learn how to position the pilot of the proportional control valve. The pilot must be positioned each time the proportional control valve is used. 6-5
In the third part of the exercise, Sensing the Speed of the Trainer Bidirectional Motor, you will learn how to sense the speed of the Bidirectional Motor using the Diffuse Reflective Photoelectric Switch and the frequency-to-voltage converter of the Signal Conditioners. In the fourth part of the exercise, Open-Loop Control of Motor Speed, you will perform open-loop control of the motor speed and see the effect of a load variation on the motor speed. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart, in Appendix A of the manual, to obtain the list of equipment required to perform this exercise. PROCEDURE Setting Up the Equipment G 1. Get the Bidirectional Motor and the Diffuse Reflective Photoelectric Switch from your storage location. G 2. Referring to Figure 6-5, position the photoelectric switch so that it is perpendicular to the motor shaft at a distance of 10 cm (4 in) (2 rows of perforation). The beam of the photoelectric switch must be pointing in the direction of the white sticker on the motor shaft. Clamp the motor into place. Figure 6-5. Photoelectric Switch Positioning. G 3. Verify the status of the trainer according to the procedure given in Appendix B. 6-6
Note: In order to obtain a motor speed which is stable, the motor should run at high speed during 1 minute. To do so, put some pneumatic oil in the motor ports and connect the circuit shown in Figure 6-6. On the Conditioning Unit, open the main shutoff valve and the required branch shutoff valve at the manifold. Set the main pressure regulator to obtain 630 kpa (90 psi) on the regulated pressure gauge. After 1 minute approximately, close the shutoff valves and turn the regulator adjusting knob completely counterclockwise. Proceed with the rest of the exercise. Figure 6-6. Circuit for lubricating the motor. G 4. Connect the circuit shown in Figure 6-7. Note: To minimize the pressure drops, use a tube as short as possible between the outlet port of the motor and the inlet port of flow control valve FCV1, and between the outlet port of flow control valve FCV1 and the muffler module. 6-7
Figure 6-7. Open-loop speed control system. G 5. Turn on the DC Power Supply and PID Controller. Do not open the shutoff valves on the Conditioning Unit at this time. 6-8
G 6. On the PID Controller, set the SETPOINT potentiometers 1 and 2 to obtain 0.0 V and 10.0 V, respectively, at the SETPOINT output 1. Then select the SETPOINT potentiometer 1. G 7. On the Conditioning Unit, open the main shutoff valve and the required branch shutoff valves at the manifold. Set the main pressure regulator to obtain 630 kpa (90 psi) on the regulated pressure gauge. G 8. Open the Flow Control Valve FCV1 completely (fully counterclockwise). G 9. On the Signal Conditioners module, connect a DC voltmeter at the output N of the f/e converter. Positioning the Pilot of the Proportional Control Valve G 10. Referring to Figure 6-8, position the pilot of the proportional control valve by screwing, or unscrewing, the pilot to obtain 1.0 V at the output N of the f/e converter. This corresponds to a speed of 1000 r/min. Do not use any tools. G 11. On the PID Controller, select the SETPOINT potentiometer 2, and set the flow control valve FCV1 to obtain 2.0 V at the output N of the f/e converter. This setting will limit the motor speed to 2000 r/min. Select the SETPOINT potentiometer 1, and ensure that the voltage at the output N of the f/e converter is still 1.0 V. Readjust the position of the pilot if necessary. Alternately select SETPOINT potentiometers 1 and 2 to verify the settings. Note: Depending on your air supply system, the speed of the pneumatic motor may vary significantly. You may need to readjust the flow control valve. 6-9
Figure 6-8. Positioning of the proportional control valve pilot Sensing the Speed of the Trainer Bidirectional Motor G 12. Select the SETPOINT potentiometer 2. Connect the DC voltmeter to the 0-10 V output of the f/e converter and calibrate this output using the potentiometer S (span) at 10.0 V. G 13. On the PID Controller, set the SETPOINT potentiometer 2 to obtain 4.0 V at the SETPOINT output 1. Measure the voltage now present at the 0-10 V output of the f/e converter. Is this voltage still 10.0 V? Why? G 14. Since the span potentiometer is adjusted to deliver 10.0 V at the 0-10 V output of the converter when the input frequency is 33 Hz (2000 r/min), calculate the actual motor speed using the voltage you measured in the previous step. Actual motor speed: r/min 6-10
Open-Loop Control of Motor Speed G 15. Open the Flow Control Valve FCV1 completely (fully counterclockwise). G 16. On the PID Controller, set the SETPOINT potentiometer 2 to obtain a motor speed of 2000 r/min. This corresponds to a voltage of 2.0 V at the output N of the f/e converter. G 17. Record the SETPOINT voltage present at the SETPOINT output 1 in the appropriate cell of Table 6-1. This condition simulates a no-load condition. LOAD CONDITION No-load Light-load SETPOINT VOLTAGE MOTOR SPEED 2000 r/min 2000 r/min Table 6-1. SETPOINT voltages for two load conditions. G 18. Simulate a speed reduction caused by an increase in motor load (or by an air supply fluctuation) by turning the knob of flow control valve FCV1 clockwise until the motor speed becomes 1800 r/min (1.8 V at the output N of the f/e converter). G 19. On the PID Controller, readjust the SETPOINT potentiometer 2 to obtain a motor speed of 2000 r/min (2.0 V at the output N of the f/e converter). Record the SETPOINT voltage present at the SETPOINT output 1 in the appropriate cell of the row Light-load of Table 6-1. G 20. Referring to the values indicated in Table 6-1, is this open-loop speed control system capable of maintaining the motor speed constant when the load conditions change? Explain. G 21. What type of control did you achieve when you increased the setpoint to compensate for the decrease in motor speed? Explain. 6-11
G 22. On the Conditioning Unit, close the shutoff valves, and turn the regulator adjusting knob completely counterclockwise. G 23. Turn off the PID Controller and the DC Power Supply. G 24. Disconnect and store all leads and components. CONCLUSION In this exercise, you learned how to measure the speed of the trainer Bidirectional Motor with a photoelectric switch and a frequency-to-voltage converter. You saw that the converter output voltage was directly proportional to the motor speed. You also performed open-loop control of the motor speed. You saw that the motor speed decreased when you increased the load. Since the system was operating in the open-loop mode, it did not receive electrical feedback to correct for the decrease in motor speed. REVIEW QUESTIONS 1. What is an open-loop speed control system? 2. What does disturbance mean? 3. Is the open-loop speed control system able to correct for variations in motor speed? Explain. 4. Should an open-loop speed control system be used in applications where the motor load varies from time to time and good speed regulation is of great importance? 6-12